There has been proposed a microscope whose relay optical system is made to be a zoom lens with keeping positional relation between a primary image and a secondary image to be constant and with keeping positional relation between an entrance pupil and an exit pupil of the relay optical system to be substantially constant, for example, see Japanese Patent publication No. 3726275.
However, in the conventional variable power relay optical system, since an exit pupil of the variable power lens system (hereinafter simply called as a “pupil of the variable power lens system”) is located inside of the variable power lens system, when an optical modulation device such as a phase plate is disposed at the position of the pupil of the variable power lens system, the optical modulation device physically interferes with the lens.